caveat: any design suggestions are based on what i thought you wrote, and a bag of unfounded assumptions. Code something that works, and refactor it until a 'good' model emerges - you'll end up doing that regardless of where you start, toso pick something that makes sense on paper or in your mind, and start with that.
caveats aside, it sounds like you have the following classes:
- Ad
- Website
- Event
- Scanner
The Scanner scans a list of web sites, looking for ads and events. The extraction regex/logic is specific to a particular web site (thus eliminating your switch statement), but if extraction is always a regex then you can pull that from a database table to instantiate web site objects as needed (or just pull the regex from a db or in-memory table).
in other words, soft-code the web site regexes if possible, so you don't have to have a subclass for each web site
don't worry about efficiency until it works and you're happy with the underlying model, then use a profiler to see where the implementation needs to be tweaked, if at all
i suspect that a 'good' model will also naturally be efficient
good luck!